§1.1 Field of the Invention
The present invention concerns wireless communications. In particular, the present invention concerns cooperative wireless communications used to improve data throughput, reliability, and/or range.
§1.2 Background Information
In a typical wireless network, each transmitter is surrounded by several other stations (or more generally, nodes or devices). However, usually, only the link between the sender device and the receiver device is used to send the data. In a fading environment, data transmission over this link might not be reliable. For example, deep fading and interference may cause signal corruption, which results in the data getting lost.
Cooperative wireless communication techniques provide a potential solution by recruiting relays or helpers. When a relay is employed, the possibility of losing data or receiving inaccurate data decreases. (See, e.g., the articles: A. Sendonaris, E. Erkip, and B. Aazhang, “User Cooperation Diversity-Part I: System Description,” IEEE Trans. on Communications, Vol. 51, No. 11, pp. 1927-1938 (November 2003) (incorporated herein by reference); A. Sendonaris, E. Erkip, and B. Aazhang, “User Cooperation Diversity—Part II: Implementation Aspects and Performance Analysis,” IEEE Trans. on Communications, Vol. 51, No. 11, pp. 1939-1948 (November 2003) (incorporated herein by reference); J. N. Laneman, G. W. Wornell, and D. N. C. Tse, “An Efficient Protocol for Realizing Cooperative Diversity In Wireless Networks,” ISIT 2001, p. 294 (June 2001) (incorporated herein by reference); and J. N. Laneman, D. Tse, and G. Wornell, “Cooperative Diversity In Wireless Networks: Efficient Protocols and Outage Behavior,” IEEE Transactions on Information Theory, Vol. 50, No. 12, (December 2004) (incorporated herein by reference).)
While cooperative communications is actively researched in the physical layer (“PHY”), modifications to the higher layer protocol stack are needed to discover and utilize all relays. Previous work by at least some of the present inventors presented a cooperative MAC protocol for IEEE 802.11. (See, e.g., the references: P. Liu, Z. Tao, and S. Panwar. “A Cooperative MAC Protocol for Wireless Local Area Networks,” IEEE Intl. Conf. on Communications (Seoul, Korea, June 2005) (incorporated herein by reference); T. Korakis, Z. Tao, Y. Slutskiy, and S. Panwar, “A Cooperative MAC Protocol for Ad-Hoc Wireless Networks”, PWN07, (White Plains, N.Y., March 2007) (incorporated herein by reference); and S. Panwar, P. Liu and Z. Tao, “Cooperative Wireless Communications”, U.S. Pat. No. 7,330,457 (incorporated herein by reference).) Each packet is transmitted over two hops, first from the source to the relay and then from the relay to destination. These references demonstrated that the network performance metric, such as throughput and delay performance can be greatly improved by using cooperation.
Typically, there might be more than one device (or node) that can “overhear” a packet sent by a source device. If such devices are willing and able to transmit cooperatively to the destination device, cooperative diversity can be much larger than in the case of a single relay. However, if all relay devices transmit sequentially in time, the time required to complete transmission increases linearly with the number of relay devices. Thus, although using more relay devices increases diversity, network throughput, which is measured by the number of bits successfully received in a unit time, may actually decrease when the system employs a lot of relay devices.
Recent advances in Multi-Input Multi-Output (“MIMO”) systems allow multiple antennas to transmit together to achieve high diversity gains using space-time coding (“STC”). Hence, multiple relay devices may be used as a distributed antenna array to mimic a MIMO system. In fact, the spatial diversity obtained by cooperation increases linearly with the number of relays. Simultaneous transmission of multiple relay devices that decode the source information at the PHY layer can be accomplished by a distributed space-time code (“DSTC”). (See, e.g., J. N. Laneman, D. Tse, and G. Wornell, “Cooperative Diversity In Wireless Networks: Efficient Protocols and Outage Behavior,” IEEE Transactions on Information Theory, Vol. 50, No. 12, (December 2004) (incorporated herein by reference).) FIG. 1 illustrates such a transmission. The basic idea is to coordinate and synchronize the relay devices so that each relay device acts as one antenna of a regular STC. (See, e.g., the articles: V. Tarokh, H. Jafarkhani, and A. Calderbank, “Space-Time Block Codes from Orthogonal Designs,” IEEE Transactions on Information Theory, Vol. 45, No. 5, pp. 1456-1467 (July 1999) (incorporated herein by reference); and S. Alamouti, “A Simple Transmit Diversity Technique for Wireless Communications,” IEEE Journal on Selected Areas in Communications, Vol. 16, No. 8, pp. 1451-1458 (October 1998) (incorporated herein by reference).)
Unfortunately, however, DSTC poses a number of challenges from a system perspective. Each relay participating in a DSTC needs to be assigned a number (and each relay must know of its number) so that it knows exactly which antenna it will mimic in the underlying STC. Further, DSTC does not fully exploit all available relay devices because even though stations other than the chosen relay devices may decode the source information correctly, they are not allowed to transmit. This sacrifices potential diversity and coding gains. Also, each individual link between the source and the relay might not be reliable due to noise, interference, and/or mobility. If one of the relay devices does not receive the signal from the source device, it cannot forward it in the next hop, which degrades the diversity gain of the system.
The application of DSTC communications in mobile ad hoc networks has been discussed in the article, G. Jakllari, S. V. Krishnamurthy, M. Faloutsos, P. V. Krishnamurthy, and O. Ercetin, “A Framework for Distributed Spatial-Temporal Communications in Mobile Ad hoc Networks,” IEEE INFOCOM, (Barcelona, Spain, April 2006) (incorporated herein by reference).
In that article, the source device can only recruit a fixed number of relay devices. The source device transmits a packet in the first hop and expects a busy tone signal from each of the relay devices sent sequentially in time. Even if a single relay device does not receive the packet from the source device, and consequently fails to respond with a busy tone, the source device transmits directly to the destination device rather than rely on the recruited relay devices.
Thus, it would be useful to improve the known DSTC systems.